Various portable optical measuring devices for sensing a bioactive signal are proposed. This device is used by being put on the arm, etc. of a subject person. It comprises a sensor which detects a bioactive signal, a signal transmission circuit which sends the signal detected by the sensor to a monitoring unit, and a battery which supplies power to the sensor and the signal transmission circuit.
When it is intended to monitor health condition of a subject person accurately, frequent detection of bioactive signal is required, which results in an increased power consumption of the device. For the optical measuring device which is put on the arm, etc. of a subject person, it is not suitable to incorporate a large battery of large power capacity. On this account, the user is required to replace the battery frequently, resulting in a troublesome operation and higher running cost.
Primary data produced by the device from the bioactive signal are data related to time, such as the number of heart beats and the intervals of heart beats. It is difficult to get data necessary for the prediction of abnormality of body, e.g., data of the absolute value of the heart beat amplitude (absolute value of the optically sensed signal) and data of the time-wise variation and small variation of the amplitude on the time axis.
To reduce the power consumption, it is proposed to construct an optical measuring device as shown in FIG. 13. This device includes a light emitting element 201 and a light sensitive element 202, which is a.c.-coupled through a capacitor 203 to an amplifier 204. The output signal of the amplifier 204 is processed by an A/D converter 205. For the output signal V1 of the light sensitive element 202, the amplifier 204 has its input signal V2 alternating across a zero d.c. voltage level and produces an amplified signal V3, as shown in FIG. 14.
It is also proposed, as shown in FIG. 15, to activate a light emitting element 301 is in a certain duty cycle so that it consumes less power for light emission. Specifically, the current supply to the light emitting element 301 is turned on and off, with the output thereof being amplified with an amplifier 303 and thereafter A/D-converted by an A/D converter 304. In this case, however, the amplifier 303 must cover in its full-scale input range Vf a ripple component which floats from zero volt as shown in FIG. 16, and a resulting small amplitude of ripple component relative to the full scale range Vf results in a poor accuracy of A/D conversion output. Namely, a small sensed signal having a large d.c. component easily exceeds the full-scale range Vf when it is amplified intact.